1. Field of the Invention:
This invention relates to a delay circuit for contour compensation processing of video signals in systems such as television receivers and television cameras and, more particularly, is directed to a delay circuit using a single delay line to produce delayed output signals which are delayed by two different amounts.
2. Description of the Prior Art:
Video signals in video systems such as television receivers and television cameras pass through various electric circuits and signal transmission lines having finite operation frequency ranges. Often, high frequency component attenuation of the signal occurs, which results in a so-called reduction of resolution. By way of example, if the luminance signal contains a 3.58-MHz color subcarrier, then a 35.8-MHz beat brilliance variation result in the reproduced image. To eliminate such beat interference, the video amplifier circuit in a color television receiver has frequency characteristics such that the color subcarrier is attenuated by more than 15 to 16 dB. Therefore, through such a video amplifier circuit the high frequency components of the video signal are attenuated, which results in the reduction of the resolution of the reproduced image. In a shadow-mask type cathode ray tube, the brilliance modulation efficiency is reduced when the frequency exceeds 2 MHz. The reduction of the brilliance modulation efficiency reduces the contrast to reduce the resolution.
To compensate for such reduction of the resolution, it has been the practice to effect contour compensation processing on portions of the luminance signal waveform corresponding to the contour of the image with an overshoot or undershoot of 20 to 30%. This processing has the effect of increasing the sharpness of the contour portions of the image, thus improving the resolution.
Heretofore, a vertical contour compensation circuit 10 as shown in FIG. 1 has been broadly used for improving the resolution of the image in the vertical direction.
In the vertical contour compensation circuit 10 shown in FIG. 1, an input luminance signal Y.sub.in, which has not been contour compensated, is coupled to a signal input terminal 1 to be fed to a delay circuit 2 and a first adder 3. The input luminance signal Y.sub.in has a waveform as shown, for instance, at A in FIG. 2. The delay circuit 2 produces a first delayed luminance signal Y.sub.DL1 having a waveform as shown at B in FIG. 2, which is delayed after the input luminance signal Y.sub.in by one horizontal scanning period 1 H, and a second delayed luminance signal Y.sub.DL2 having a waveform as shown at C in FIG. 2, which is delayed after the input luminance signal Y.sub.in by 2 H. The first delayed luminance signal Y.sub.DL1 obtained from the delay circuit 2 is fed to a subtracter 5 and a second adder 7. The second delayed luminance signal Y.sub.DL2, on the other hand, is fed to the first adder 3. The first adder 3 adds the input luminance signal Y.sub.in and the second delayed luminance signal Y.sub.DL 2 and feeds the resultant signal Y.sub.A, (having a waveform as shown at D in FIG. 2,) through an attenuator 4 to the subtracter 5. The subtracter 5 subtracts the resultant signal Y.sub.A from the first delayed luminance signal Y.sub.DL1 to obtain a contour compensation signal S.sub.AC having a waveform as shown at E in FIG. 2. This contour compensation signal S.sub. AC is fed through a level controller 6 to the second adder 7. The second adder 7 superimposes the contour compensation signal S.sub.AC on the first delayed luminance signal Y.sub.DL1 and produces an output luminance signal Y.sub.out as shown at F in FIG. 2. The signal Y.sub.out appears at an output terminal 8 and has a vertical contour compensated waveform with an overshoot and an undershoot generated as a result of the superimposition of the contour compensation signal S.sub.AC on the luminance change portions, i.e., contour portions in the vertical direction of the image.
The delay circuit 2 in the vertical contour compensation circuit 10 usually requires two 1 H delay lines 22 and 25, each providing a delay time equal to 1 H. Referring to FIG. 1, the input luminance signal Y.sub.in coupled to the input terminal 1 is fed to a modulator 21 in the delay circuit 2 for amplitude modulation. The output from modulator 21 is fed to the first delay line 22. The delayed output signal from the first delay line 22 is fed through a first gain controlled amplifier 23 to a first demodulator 24. The first gain controlled amplifier 23 is gain controlled by the output of the first demodulator 24. The delayed output signal from the first delay line 22 is fed through the first gain controlled amplifier 23 to the second delay line 25. The delayed output signal from the second delay line 25 is fed through a second gain controlled amplifier 26 to a second demodulator 27.
When the input luminance signal Y.sub.in, as shown at A in FIG. 2, is fed to the amplitude modulator 21, the first demodulator 24 demodulates the 1 H delayed luminance signal from the first delay line 22 and produces the first delayed luminance signal Y.sub.DL1, which is delayed by 1 H behind the input luminance signal Y.sub.in, as shown at B in FIG. 2. The second demodulator 27 demodulates the luminance signal that has been delayed by 1 H through each of the first and second delay lines 22 and 25 and produces the second delayed luminance signal Y.sub.DL2 delayed by 2 H behind the input luminance signal Y.sub.DL2, as shown at C in FIG. 2.
As has been shown, the delay circuit 2 in the vertical contour compensation circuit 10 according to the prior art uses two 1 H delay lines 22 and 25 in order to obtain the first and second delayed luminance signals Y.sub.DL1 and Y.sub.DL2 delayed by 1 H and 2 H behind the input luminance signal Y.sub.in, respectively. Since a delay line is generally expensive, the vertical contour compensation circuit 10 using two high performance 1 H delay lines to provide a comparatively long delay time and which also has wide frequency band characteristics, is inevitably very expensive. The major proportion of the price is occupied by the delay lines 22 and 25. In a small addition, the signal level attenuation and temperature characteristics vary with individual delay lines having the same ratings and specifications. Therefore, where the two delay lines 22 and 25 are used, as in the prior art, the delayed output signals must be passed through the gain controlled amplifiers 23 and 26 for the AGC level control.